A solid laser, semiconductor laser and gas laser having a large output and a small size, which radiate an ultraviolet ray, visible light or infrared ray having a wavelength of 300 to 1,200 nm, have become easily available, and these lasers are very useful for recording light sources used in the direct plate making based on digital signals, for example, from a computer. Various investigations on recording materials sensitive to such various laser beams have been made. Typical examples thereof include firstly recording materials capable of being recorded with a infrared laser having a wavelength of not less than 760 nm, for example, positive type recording materials described in U.S. Pat. No. 4,708,925 and acid catalyst crosslinking type negative type recording materials described in JP-A-8-276558 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”), and secondly recording materials responsive to an ultraviolet ray or visible light laser having a wavelength of from 300 to 700 nm, for example, radical polymerization type negative type recording materials described in U.S. Pat. No. 2,850,445 and JP-B-44-20189 (the term “JP-B” as used herein means an “examined Japanese patent publication”).
Further, with respect to hitherto known lithographic printing plate precursors (hereinafter, also referred to as PS plates), a step of removing the non-image area by dissolution (development processing) is indispensable and a post-processing step, for example, washing the printing plate after the development processing with water, treatment of the printing plate after the development processing with a rinse solution containing a surfactant or treatment of the printing plate after the development processing with an oil-desensitizing solution containing gum arabic or a starch derivative, is also necessary. The point that such additional wet treatments are indispensable is a large subject of investigation in hitherto known PS plates. Even when the first half (image-forming process) of plate making process is simplified by the above-described digital processing, the effects due to the simplification is still insufficient as long as the last half (development processing) is the troublesome wet treatment.
Particularly, the consideration for global environment has become a great concern throughout the field of industry in recent years. In view of the consideration for global environment, a treatment with a developer closer to a neutral range and a small amount of waste liquid are subjects of further investigations. Further, it is desirable that the wet type post-processing is simplified or changed to a dry processing.
From this viewpoint, as one method for eliminating the processing step, a method referred to as on-machine development wherein an exposed printing plate precursor is mounted on a cylinder of a printing machine and the non-image area of the printing plate precursor is removed by supplying dampening water and ink while rotating the cylinder is known. Specifically, according to the method, the printing plate precursor is exposed and mounted on a printing machine as it is to complete development processing in a conventional process of printing.
A lithographic printing plate precursor suitable for the on-machine development is required to have an image-forming layer soluble in dampening water or an ink solvent and a bright room handling property suitable for development on a printing machine placed in a bright room. However, it is substantially impossible for hitherto known PS plates to fulfill such requirements.
In order to fulfill such requirements, a lithographic printing plate precursor having provided on a hydrophilic support an image-forming layer in which fine particles of thermoplastic hydrophobic polymer are dispersed in a hydrophilic binder polymer is proposed (see, for example, Japanese Patent 2,938,397). In the plate-making, the lithographic printing plate precursor is exposed to an infrared laser to agglomerate (fuse) the fine particles of thermoplastic hydrophobic polymer by heat generated by light-to-heat conversion thereby forming an image, and mounted on a cylinder of a printing machine to carry out on-machine development by supplying at least any one of dampening water and ink. Since the lithographic printing plate precursor has the sensitive zone in an infrared region, it has also the handling property in a bright room.
However, the image formed by the agglomeration (fusion) of the fine particles of thermoplastic hydrophobic polymer is insufficient in strength and has a problem of printing durability as a printing plate.
Lithographic printing plate precursors including microcapsules containing a polymerizable compound incorporated therein in stead of the thermoplastic fine particles are also proposed (see, for example, JP-A-2000-211262, JP-A-2001-277740, JP-A-2002-29162, JP-A-2002-46361, JP-A-2002-137562, JP-A-2002-326470 and JP-A-2006-15530). In the lithographic printing plate precursors according to such a proposal, it is advantageous that the polymer image formed by a reaction of the polymerizable compound is excellent in the strength in comparison with the image formed by the fusion of the fine particles.
Also, since the polymerizable compound has high reactivity, many proposals for isolation of the polymerizable compound using microcapsules have been made. Further, it has been proposed to use a thermally degradable polymer in a shell of the microcapsule.
However, in the hitherto known lithographic printing plate precursors described in Japanese Patent 2,938,397, JP-A-2000-211262, JP-A-2001-277740, JP-A-2002-29162, JP-A-2002-46361, JP-A-2002-137562 and JP-A-2002-326470, printing durability of the image formed by laser exposure is insufficient so that further improvements have been requested. Specifically, in such a lithographic printing plate precursor of a simple processing type, a support having a surface of high hydrophilicity is used in order to make possible development with an aqueous solution having pH of 10 or less or dampening water (ordinarily nearly neutral) on a printing machine and as a result, the image area is apt to be removed from the support by dampening water during printing so that sufficient printing durability can not be obtained. On the contrary, when the surface of support renders hydrophobic, ink also adheres on the non-image area during printing to cause printing stain. Thus, it is extremely difficult to achieve a good balance between the printing durability and the stain resistance and a lithographic printing plate precursor of a simple processing type which provides good stain resistance and sufficient printing durability has not yet been known.
On the other hand, in JP-A-2006-15530, a lithographic printing plate precursor comprising a support having thereon (1) a polymer resin layer having a hydrophilic substituent capable of adsorbing the support and (2) an image-recording layer capable of being removed with an oily ink and an aqueous component in this order, wherein a contact angle of the polymer resin layer after coated on the support is from 50 to 100° is described. It is described in JP-A-2006-15530 that since the polymer resin layer is almost removed without residue at the time of removing the unexposed area by supplying an oily ink and an aqueous component on a printing machine, good stain resistance can be obtained.